Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 6
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If the layer meets specifications, the work platform is lowered a distance equal to the required layer thickness and the next layer isdeposited. However, if a clot is detected in either nozzle, a jet cleaningcycle is initiated to clear it. Then the faulty layer is milled off and thatlayer is redeposited. After the 3D model is completed, the wax materialis either melted from the object by radiant heat or dissolved away in a hotwater wash.The BPM system is capable of producing objects with fine finishes,but the process is slow. With this RP method, a slower process that yieldsa 3D model with a superior finish is traded off against faster processesthat require later manual finishing.The version of the BPM system shown in Figure 8 is called Drop onDemand Inkjet Plotting by Sanders Prototype Inc, Merrimac, NewHampshire.
It offers the ModelMaker II processing equipment, whichproduces 3D models with this method. AeroMet Corporation builds titanium parts directly from CAD renderings by fusing titanium powderwith an 18-kW carbon dioxide laser, and 3D Systems of Valencia,IntroductionCalifornia, produces a line of inkjet printers that feature multiple jets tospeed up the modeling process.Directed Light Fabrication (DLF)The Directed Light Fabrication (DLF) process, diagrammed in Figure 9,uses a neodymium YAG (Nd:YAG) laser to fuse powdered metals tobuild 3D models that are more durable than models made from paper orplastics.
The metal powders can be finely milled 300 and 400 seriesstainless steel, tungsten, nickel aluminides, molybdenum disilicide, copper, and aluminum. The technique is also called Direct-Metal Fusing,Laser Sintering, and Laser Engineered Net Shaping (LENS).The laser beam under X-Y computer control fuses the metal powderfed from a nozzle to form dense 3D objects whose dimensions are said tobe within a few thousandths of an inch of the desired design tolerance.DLF is an outgrowth of nuclear weapons research at the Los AlamosNational Laboratory (LANL), Los Alamos, New Mexico, and it is still inthe development stage. The laboratory has been experimenting with theFigure 9 Directed Light Fabrication (DLF): Fine metal powder is distributed on an X-Ywork platform that is rotated under computer control beneath the beam of a neodymiumYAG laser.
The heat from the laser beam melts the metal powder to form thin layers of a3D model or prototype. By repeating this process, the layers are built up and bonded tothe previous layers to form more durable 3D objects than can be made from plastic.Powdered aluminum, copper, stainless steel, and other metals have been fused to makeprototypes as well as practical tools or parts that are furnace-fired to increase their bondstrength.xxixxxxIntroductionlaser fusing of ceramic powders to fabricate parts as an alternative to theuse of metal powders. A system that would regulate and mix metal powder to modify the properties of the prototype is also being investigated.Optomec Design Company, Albuquerque, New Mexico, hasannounced that direct fusing of metal powder by laser in its LENSprocess is being performed commercially.
Protypes made by this methodhave proven to be durable and they have shown close dimensional tolerances.Research and Development in RPMany different RP techniques are still in the experimental stage and havenot yet achieved commercial status. At the same time, practical commercial processes have been improved. Information about this research hasbeen announced by the laboratories doing the work, and some of theresearch is described in patents. This discussion is limited to two techniques, SDM and Mold SDM, that have shown commercial promise.Shape Deposition Manufacturing (SDM)The Shape Deposition Manufacturing (SDM) process, developed at theSDM Laboratory of Carnegie Mellon University, Pittsburgh,Pennsylvania, produces functional metal prototypes directly from CADdata. This process, diagrammed in Figure 10, forms successive layers ofmetal on a platform without masking, and is also called solid free- form(SFF) fabrication.
It uses hard metals to form more rugged prototypesthat are then accurately machined under computer control during theprocess.The first steps in manufacturing a part by SDM are to reorganize ordestructure the CAD data into slices or layers of optimum thickness thatwill maintain the correct 3D contours of the outer surfaces of the part andthen decide on the sequence for depositing the primary and supportingmaterials to build the object.The primary metal for the first layer is deposited by a process calledmicrocasting at the deposition station, Figure 10(a).
The work is thenmoved to a machining station (b), where a computer-controlled millingmachine or grinder removes deposited metal to shape the first layer ofthe part. Next, the work is moved to a stress-relief station (c), where it isshot- peened to relieve stresses that have built up in the layer. The workis then transferred back to the deposition station (a) for simultaneousdeposition of primary metal for the next layer and sacrificial supportIntroductionFigure 10 Shape Deposition Manufacturing (SDM): Functional metal parts or tools canbe formed in layers by repeating three basic steps repetitively until the part is completed.Hot metal droplets of both primary and sacrificial support material form layers by a thermal metal spraying technique (a). They retain their heat long enough to remelt theunderlying metal on impact to form strong metallurgical interlayer bonds. Each layer ismachined under computer control (b) and shot-peened (c) to relieve stress buildupbefore the work is returned for deposition of the next layer.
The sacrificial metal supportsany undercut features. When deposition of all layers is complete, the sacrificial metal isremoved by acid etching to release the completed part.metal. The support material protects the part layers from the depositionsteps that follow, stabilizes the layer for further machining operations,and provides a flat surface for milling the next layer.
This SDM cycle isrepeated until the part is finished, and then the sacrificial metal is etchedaway with acid. One combination of metals that has been successful inSDM is stainless steel for forming the prototype and copper for formingthe support structureThe SDM Laboratory investigated many thermal techniques fordepositing high-quality metals, including thermal spraying and plasmaor laser welding, before it decided on microcasting, a compromisebetween these two techniques that provided better results than eithertechnique by itself. The metal droplets in microcasting are large enough(1 to 3 mm in diameter) to retain their heat longer than the 50-mmdroplets formed by conventional thermal spraying.
The larger dropletsremain molten and retain their heat long enough so that when theyimpact the metal surfaces they remelt them to form a strong metallurgical interlayer bond. This process overcame the low adhesion and lowmechanical strength problems encountered with conventional thermalmetal spraying. Weld-based deposition easily remelted the substratexxxixxxiiIntroductionmaterial to form metallurgical bonds, but the larger amount of heat transferred tended to warp the substrate or delaminate it.The SDM laboratory has produced custom-made functional mechanical parts and has embedded prefabricated mechanical parts, electroniccomponents, electronic circuits, and sensors in the metal layers duringthe SDM process. It has also made custom tools such as injection moldswith internal cooling pipes and metal heat sinks with embedded copperpipes for heat redistribution.Mold SDMThe Rapid Prototyping Laboratory at Stanford University, Palo Alto,California, has developed its own version of SDM, called Mold SDM,for building layered molds for casting ceramics and polymers.
MoldSDM, as diagrammed in Figure 11, uses wax to form the molds. The waxoccupies the same position as the sacrificial support metal in SDM, andwater-soluble photopolymer sacrificial support material occupies andsupports the mold cavity. The photopolymer corresponds to the primarymetal deposited to form the finished part in SDM. No machining is performed in this process.The first step in the Mold SDM process begins with the decomposition of CAD mold data into layers of optimum thickness, which dependson the complexity and contours of the mold. The actual processingbegins at Figure 11(a), which shows the results of repetitive cycles of thedeposition of wax for the mold and sacrificial photopolymer in eachlayer to occupy the mold cavity and support it. The polymer is hardenedby an ultraviolet (UV) source. After the mold and support structures arebuilt up, the work is moved to a station (b) where the photopolymer isremoved by dissolving it in water.
This exposes the wax mold cavity intowhich the final part material is cast. It can be any compatible castablematerial. For example, ceramic parts can be formed by pouring a gelcasting ceramic slurry into the wax mold (c) and then curing the slurry.The wax mold is then removed (d) by melting it, releasing the “green”ceramic part for furnace firing. In step (e), after firing, the vents andsprues are removed as the final step.Mold SDM has been expanded into making parts from a variety ofpolymer materials, and it has also been used to make preassembledmechanisms, both in polymer and ceramic materials.For the designer just getting started in the wonderful world of mobilerobots, it is suggested s/he follow the adage “prototype early, prototypeoften.” This old design philosophy is far easier to use with the aid of RPtools.
A simpler, cheaper, and more basic method, though, is to useIntroduction xxxiiiFigure 11 Mold Shape Deposition Manufacturing (MSDM): Casting molds can beformed in successive layers: Wax for the mold and water-soluble photopolymer to support the cavity are deposited in a repetitive cycle to build the mold in layers whose thickness and number depend on the mold’s shape (a).
UV energy solidifies the photopolymer.The photopolymer support material is removed by soaking it in hot water (b). Materialssuch as polymers and ceramics can be cast in the wax mold. For ceramic parts, a gelcasting ceramic slurry is poured into the mold to form green ceramic parts, which are thencured (c). The wax mold is then removed by heat or a hot liquid bath and the greenceramic part released (d).
After furnace firing (e) any vents and sprues are removed.Popsicle sticks, crazy glue, hot glue, shirt cardboard, packing tape, clay,or one of the many construction toy sets, etc. Fast, cheap, and surprisingly useful information on the effectiveness of whatever concept hasbeen dreamed up can be achieved with very simple prototypes. There’snothing like holding the thing in your hand, even in a crude form, to seeif it has any chance of working as originally conceived.Robots can be very complicated in final form, especially those that doreal work without aid of humans. Start simple and test ideas one at a time,then assemble those pieces into subassemblies and test those.
Learn asmuch as possible about the actual obstacles that might be found in theenvironment for which the robot is destined. Design the mobility systemto handle more difficult terrain because there will always be obstacles thatwill cause problems even in what appears to be a simple environment.Learn as much as possible about the required task, and design the manipulator and end effector to be only as complex as will accomplish that task.Trial and error is the best method in many fields of design, and isespecially so for robots.
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